Quantification of Fly Ash in Concrete by Image Analysis Techniques: a Preliminary Investigation
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QUANTIFICATION OF FLY ASH IN CONCRETE BY IMAGE ANALYSIS TECHNIQUES: A PRELIMINARY INVESTIGATION RAY E. FERRELL, JR.* and ALLISON W. DREW
Basin Research Institute, Louisiana State University, Baton Rouge, LA 70803 * Also affiliated with the Department of Geology and Geophysics, LSU Received 28 October, 1988; refereed
ABSTRACT The fly ash present in concrete mixtures is readily identified with qualitative electron microscope techniques. Quantification of the fly ash present in samples is more difficult. Visual analysis of a representative sample volume can be accomplished in a reasonable amount of time through use of image analysis techniques. Our research efforts have focused on the development of an image analysis technique which uses particle shape and composition to quantify the fly ash present in samples. The resultant binary image is filtered to eliminate the smallest particles and pieces of particles. Shape factor analysis distinguishes spherical fly ash particles from irregularly shaped grains of unhydrated cement clinker. The volume percent of nearly spherical particles is calculated and adjusted to compare the percentage of spherical particles in the matrix of fly ash-containing specimens to the percentage in a control specimen with no fly ash. There is a 2.2-fold increase in the number of spherical particles associated with the substitution of Portland cement by 60 wt% fly ash. Proportional changes in this ratio should be observable in samples with varying amounts of fly ash. The most obvious limitation of the method is related to the requirement that one knows the percentage of high atomic number particles in the fly ash beforehand.
INTRODUCTION Fly ash has been used as a partial substitute for cement in concrete mixtures for nearly half a century. Concretes having variable ash to cement ratios have been used successfully by the construction industry [1,2], although replacement more commonly ranges from 10 to 50 wt% fly ash for cement. The amount of fly ash substituted for cement in a given concrete is dependent on the physical and chemical properties of the fly ash being used, and the desired properties of the final product [3,4]. For example, Class C, or high-calcium, fly ashes exhibit cementitious properties (producing hydration products by direct reaction of ash compounds and water), and strength development is largely independent of cement content. Larger amounts of Class C fly ash can be substituted for cement before deleterious effects are observed. The demand for different quantities and types of fly ash for different projects illustrates the need for a simple, effective means of quantifying the fly ash component of concretes. Approaches considered by previous researchers include separation of the characteristic denser constituents of fly ash from the concrete mix by the use of heavy liquids [5], and measurement of variations in the concentration of distinctive mineral phases (i.e., mullite) attributable to fly ash in the cement mix [6]. The high toxicity of heavy liquids and the difficulty of
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